1. Field of the Invention
The present invention relates to a fixing device which fixes an image onto a recording medium, and relates also to an image forming apparatus including the fixing device.
2. Description of the Related Art
In recent years, marketing needs are increasing for energy saving purposes and high-speed performance in an image forming apparatus, such as a printer, a copier, and a facsimile. In the image forming apparatus, an unfixed toner image is formed on a recording medium, such as a recording medium sheet, a printing sheet of paper, a photosensitive sheet of paper, an electrostatic recording sheet of paper. At this time, the image formation is performed using an image transfer method or a direct method, through an image forming process, including electrophotographic recording, electrostatic recording, or magnetic recording. As a fixing device for fixing an unfixed toner image, the widely applied system is a fixing device of a contact heating system, for example, a heat roller system, a film heating system, and an electromagnetic induction heating system.
An example of this fixing device is a known fixing device with a so-called belt system (for example, see Japanese Patent Application Laid-open No. 2010-72124 and Japanese Patent Application Laid-open No. 2004-286922).
In recent years, there is a demand on energy saving performance, and there is also a demand on foreshortening of a warm-up time and a fast printing time. Note that the warm-up time indicates the time of power activation, for example, the time required for a predetermined temperature (reload temperature) from a normal temperature, while the fast printing time indicates a period of time since reception of a print request until complete discharge of the printed sheet of paper after a printing operation via print preparation. To meet these demands, low heat capacity is attempted using a fixing member which is formed of a thin wall roller or an endless-loop belt member (including a film-like form) with a flexible thin wall. Also, high speed heating is realized using a heat source, such as a halogen heater or a graphite heater, which heats the fixing member with radiant heat, a ceramic heater, and an IH system with high heating efficiency. The heating method may be a heating method for indirectly heating the fixing member through a metal conductor using a heat source (for example, see Japanese Patent Application Laid-open No. 2007-334205) or a heating method for directly heating the fixing member using a heat source (for example, see Japanese Patent Application Laid-open No. 2002-49264, Japanese Patent Application Laid-open No. 2010-217205, Japanese Patent Application Laid-open No. 2007-233011, Japanese Patent No. 2861280, and Japanese Patent Application Laid-open No. 2011-158558). As the method for directly heating the fixing member using a heat source, an applicable method may be a method for heating an area of a nip of the fixing member using a heat source (see Japanese Patent No. 2861280 and Japanese Patent Application Laid-open No. 2011-158558) or a method for heating an area other than the nip of the fixing member using a heat source (see Japanese Patent Application Laid-open No. 2002-49264, Japanese Patent Application Laid-open No. 2010-217205, and Japanese Patent Application Laid-open No. 2007-233011).
In this type of fixing device, to secure the stable fixity maintained at a target temperature (fixing temperature) of the fixing member, a temperature-detecting unit detects the temperature of the fixing member based on temperature information from the temperature-detecting unit, and turning-on/off of the heat source is controlled. For example, in Japanese Patent Application Laid-open No. 2010-72124, the temperature information from the temperature-detecting unit for detecting the temperature of the fixing belt is input to a temperature controller (PID controller), and the temperature controller obtains a duty based on a difference between the temperature detected by the temperature-detecting unit and the target temperature. This duty is also called a duty ratio, and is a ratio W/C of a turn-on time period W per unit time (per control cycle C) in the turn-on/off cycle of a heat source). The controller controls the turning-on/off of the heat source through a PMM driving circuit, based on the obtained duty. As a result, the heat source repeats operating/stopping in accordance with the duty. When the heat source is a halogen heater, the halogen heater repeats the turning-on/off in accordance with the duty.
As described above, in the conventional fixing device, the turning-on/off of the heat source is controlled in accordance with a duty for a predetermined control cycle, based on the temperature information from the temperature-detecting unit which detects the temperature of the fixing belt. By doing this, the temperature of the fixing member is kept at a target temperature (fixing temperature), to attain the stable fixity. However, in the fixed image on the sheet of paper, the gloss value varies, and image irregularities occur in accordance with the progress of the continuous image-forming job, in the continuous image-forming job for continuously forming images on a plurality of recording media, thus lowering quality of toner fixing.
Therefore, there is a need for a fixing device and an image forming apparatus capable of preventing lowering of fixing quality in a continuous image-forming job.